This invention relates to strain gauges and more particularly to an optical strain gauge utilizing evanescent field coupling between optical fibers.
Light propagating in an input optical fiber will couple into an output fiber at a site where the two fibers are closely spaced through the mechanism known as evanescent field coupling. The optical electromagnetic field outside an optical fiber falls off exponentially from the outer wall of the fiber. Thus, the amount of light energy coupled into the output fiber is a function of the separation between the two fibers at a site where the two fibers are in close proximity.
It is therefore an object of this invention to provide a highly accurate strain gauge based on the principle of evanescent field coupling.
Another object of the invention is such an optical strain gauge which is capable of measuring strain substantially simultaneously at a plurality of locations in an extended mechanical structure such as an airframe or a bridge.
A still further object of the invention is an optical strain gauge employing but a single input and a single output optical fiber to measure strain at a plurality of locations.